1. Field of the Invention
The present invention relates generally to a visible light communication system, and more particularly to an apparatus and a method for transmitting and receiving an information symbol in a visible light communication system for a Color Code Modulation (CCM) scheme using a chromaticity diagram.
2. Description of the Related Art
A visible light communication system wirelessly transmits data using inside/outside lighting, a signboard, a traffic light, a streetlight, etc., using a Light Emitting Diode (LED). For example, visible light communication systems may be implemented in a hospital or an airplane, i.e., areas in which the use of Radio Frequency (RF) is normally restricted, and may also be used to provide additional product or service information using a signboard. Further, the visible light communication system may also be implemented in a Back Light Unit (BLU) of a Liquid Crystal Display (LCD) television, and used for high-speed data transmission because its frequency band is not limited under the Federal Communications Commission (FCC).
A visible light communication system emits visible lights rays corresponding to a blue, green, red, and white color of the LED through the CCM using a chromaticity diagram. The chromaticity diagram is a diagram illustrated according to the color specification established in a general meeting of Commission Internaitonale del'Eclairage in 1931.
According to the chromaticity diagram, every color is indicated with three parameters, i.e., x, y, and Y, measured by a spectrophotometer. Here, Y is light metering quantity that represents color brightness, and x and y represent the chromaticity. Chromaticity is a property of a color, except for its brightness (or luminance), and is represented as a center point in the x and y axis on the chromaticity diagram. By indicating the chromaticity of monochromatic light of each wavelength as a point on the chromaticity diagram, the indicated points being connected in a line, and the chromaticity points of pure purple and pure red-purple being connected, a horseshoe-shaped figure is generated on the chromaticity diagram including every color.
FIG. 1 is a chromaticity diagram illustrating a constellation used in the conventional CCM and FIG. 2 is a chromaticity diagram illustrating a constellation arbitrarily moved in the conventional CCM. More specifically. FIGS. 1 and 2 illustrate a constellation in Quadrature Phase Shift Keying (QPSK) among the modulation schemes. The constellation in the present application refers to a point indicated according to a coordinates value corresponding to the modulation scheme on the chromaticity diagram.
Referring to FIG. 1, a constellation is located as symbols in a band displaying the color white, in order to solve the color balancing problem.
The communication performance (e.g., bit error rate) in the visible light communication system is improved as each point of the constellation moves farther away from each other on the chromaticity diagram. However, as illustrated in FIG. 2, if the constellation is arbitrarily moved, the distance between the points of the constellation increases, but the color between each point of the constellation is not balanced and fails to display the color white. Therefore, this deteriorates the advantage of the simultaneous implementation of the light and communication of the visible light communication system.
Further, if 8 PSK, 16 QAM (Quadrature Amplitude Modulation), 32 QAM, or 64 QAM modulation scheme is used for the high-speed data transmission, by using the constellation arbitrarily moved in the conventional CCM, color balancing cannot be maintained.
Further, if a high order modulation is used in each point of the conventional constellation to maintain the color balancing through using the constellation on the chromaticity diagram, modulation performance is degraded in comparison with a general high order modulation. That is, if the distance between each point of the constellation increases for improving the communication performance in the CCM of the conventional visible light communication system, this causes a problem in color balancing.
Accordingly, a need exists for a method capable of using high order modulation while maintaining color balancing when the distance between each point of a constellation is increased to improve communication performance in a CCM of a visible light communication system.